It has been suggested to form deep trenches in semiconductor substrates and then to refill the trenches in order to provide isolation between active regions or devices in the substrate. For instance, deep trenches can be formed according to the procedures disclosed in U.S. Pat. No. 4,256,514 to Pogge. The refill techniques employed in deep trench formations have been chemical vapor deposition (i.e. CVD) systems. Such systems involve a homogeneous gas phase reaction; whereby the silicon dioxide or polycrystalline silicon formed in the gas from the reactive gas species present, essentially "drop" onto surfaces and into the trench structures. However, such process suffers from a serious reliability problem in that there is a tendency to form large voids within the trench. In fact, frequently voids are formed at the bottom of the trenches and/or large continuous openings are formed extending to the final surface after subsequent planarization. Moreover, the refill deposited by such procedure can be structurally deficient (e.g., loosely packed) which can lead to problems in future processing. The presence of voids has a tendency to magnify the formation of defects in silicon areas later to serve as active or device regions. This is particularly so if the formation continues up to the surface followed by subsequent oxidation or isolation. Moreover, substrate surfaces which are relatively densely populated with trenches in some areas and less densely populated in others have been found to display local deposition thickness variations. This is due to depletion effects arising from the presence of increased area. These overall surface variations can cause problems in later planarization steps.